1. Technical Field
The present disclosure is related to a narrow band excimer laser apparatus.
2. Related Art
Accompanying miniaturization and increased levels of integration of semiconductor integrated circuits, improvement of resolving power is desired in semiconductor exposure apparatuses. Hereinafter, semiconductor exposure apparatuses will simply be referred to as “exposure apparatuses”. For this reason, the wavelength of light output from exposure light sources is being shortened. Gas laser apparatuses are being used in place of conventional mercury lamps as light sources for exposure. Currently, a KrF excimer laser apparatus that outputs ultraviolet light with a wavelength of 248 nm and an ArF excimer laser apparatus that outputs ultraviolet light with a wavelength of 193 nm are being employed as gas laser apparatuses for exposure.
Immersion exposure, in which a gap between a projection lens on the side of an exposure apparatus side and a wafer is filled with a liquid and the refractive index of the gap is changed to shorten the apparent wavelength of the exposure light source, is in practical use as a current exposure technique. When immersion exposure is performed using an ArF excimer laser apparatus as a light source for exposure, the wafer is irradiated with ultraviolet light having a wavelength of 134 nm in water. This technique is referred to as ArF liquid immersion exposure. ArF liquid immersion exposure is also referred to as ArF liquid immersion lithography.
The spectral linewidth in natural oscillation of the KrF or ArF excimer laser device is as wide as approximately 350 to 400 pm. Therefore, chromatic aberration of the laser light (ultraviolet light), which is reduced and projected onto the wafer by the projection lens on the exposure apparatus side, is generated, and resolving power decreases. Accordingly, it is necessary to narrow the spectral line width of the laser light output from the gas laser device until the chromatic aberration becomes negligible. Spectral linewidth is also referred to as spectral width. For this reason, a band narrowing module (Line Narrowing Module) having a band narrowing element is provided in a laser resonator of the gas laser apparatus, and narrowing of spectrum width is realized by this band narrowing module. Note that the band narrowing element may be an etalon, a grating or the like. Such a laser apparatus in which the spectrum width is narrowed is called a narrow band laser apparatus.